1. Field of the Invention
The present invention is directed generally to a window. More specifically, the present invention is directed to a window over a sensor which provides radio frequency signature management (stealth) capabilities while maintaining field of regard for the sensor.
2. Background Information
Platforms outfitted with sensors require windows and external surfaces to house, allow transmission and reception, and protect sensors. Such windows can be covered apertures through which a desired frequency or wavelength of energy is transmitted or received or can be openings in the external covering of the platform.
It has become desirable to build and equip platforms which have a minimum radar or radio frequency signature. This characteristic, also called stealth technology, has been utilized, for example, by the military in an attempt to minimize the detection of aircraft. In a typical radar or radio frequency interaction, the transmitted RF energy intercepts and reflects off surfaces of a platform, returning to a receiver which senses and translates the return signal to an operator and indicates that the platform has been detected. Typical radars can operate in the submicron to millimeter wave range.
Radio frequency signature management (RFSM), or stealth technologies, attempt to minimize the radio frequency energy interaction with the platform. In one example, the angularity and the materials selected for such applications either reflect, refract or absorb the radio frequency energy, thus minimizing or substantially preventing a return signal to the receiver.
Although the application of radio frequency signature management systems to platforms have been successful, the application of radio frequency signature management systems to components of the platform, such as the windows over sensors, have produced more limited successes. For example, prior windows have been of planar or single pane design. Such windows have been positioned conformally to the body of the platform. The materials of the sensor window are not typically adequate to manage the radio frequency signature without enhancements. An overlay material has been disposed over the surface of the conformal window to provide radio frequency and energy management.
However, the use of conformal windows adversely impacts the performance of the sensor utilizing the window, such as the field of regard. Field of regard (FOR) refers to the search volume available to the sensor. Field of Regard is different from Field of View (FOV), which refers to the angle subtended by the optical elements of a sensor. Limiting the Field of Regard deleteriously impacts the performance of the sensor by limiting the search volume, and limiting the tactical and situational usefulness of the sensor. For example, a limited Field of Regard can result in the inability to maintain continuous searching and tracking of a desired target.
Therefore, it is desirable to have a window for a sensor mounted on a platform which provides the required radio frequency signature management and aerodynamic performance while also providing an expanded field of regard.
A window has at least two surfaces oriented relative to each other at an angle greater than zero. The surfaces meet at an edge and are transparent to at least a first predetermined bandwidth of energy, such as a wavelength of the platform""s sensor. A conductive path is disposed across the two surfaces and renders the window reflective or absorptive to a second predetermined bandwidth of energy, which can be radio frequency energy.
The window signature to the predetermined bandwidth of energy is a function of the angle formed by the intersection of the at least two planar surfaces and the line of the angle.
The window can be a plurality of segmented panes or a unitary body. Suitable materials for the window are IR transmissive and include sapphire or chalcogenide glass.
The window provides a conductive path that is in electrical contact with the host platform. The conductive path can be gold, platinum, titanium, a noble metal, or a material that is impedance matched to a host platform. The conductive path can be deposited on the surface using any suitable technique, such as by electrodeposition, electrodeless deposition, physical vapor deposition techniques, or chemical vapor deposition techniques. In an exemplary embodiment, the conductive path is a single or overlaying grid pattern of shapes, the sizes of which are chosen to obtain a desired sheet resistivity.
A method to form a window is also provided. A plurality of segments are faceted with a desired mitered edge and a conductive surface treatment is deposited on a first surface. The segments are joined by a structural adhesive and a conductive adhesive to form a surface, as delineated by the first surfaces of each of the segments. A conductive connector at the joint of the segments of the surface establishes electrical continuity between the host platform and the conductive surface treatment of each of the segments, thereby providing an electrical signature of the window that is impedance matched to the electrical signature of the host platform.